NCERT Class 11 Geography Chapter 9 Solar Radiation, Heat Balance and Temperature

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NCERT Class 11 Geography Chapter 9 Solar Radiation, Heat Balance and Temperature

Also, you can read the NCERT book online in these sections Solutions by Expert Teachers as per Central Board of Secondary Education (CBSE) Book guidelines. CBSE Class 11 Geography Solutions are part of All Subject Solutions. Here we have given NCERT Class 11 Geography Part I: Fundamentals of Physical Geography, Part II: Indian: Physical Environment, Part III: Practical Work in Geography. NCERT Class 11 Geography Chapter 9 Solar Radiation, Heat Balance and Temperature Notes, NCERT Class 11 Geography Textbook Solutions for All Chapters, You can practice these here.

Solar Radiation, Heat Balance and Temperature

Chapter: 9

GEOGRAPY [ PART – I ]

II. Fill in the blanks:

(i) The incoming solar radiation is called ____________.

Ans. Insolation.

(ii) The outgoing heat energy from surface of the earth is called ____________.

Ans. Terrestrial radiation.

(iii) The balance of incoming and outgoing radiation is called ____________.

Ans. Heat Budget.

(iv) Intensity of heat is called ____________.

Ans. Radiation.

(v) The imaginary lines having the equal temperature reduced to sea level are called ____________.

Ans. Isotherms.

III. Make correct pairs from the following two columns.

(i) Insolation	(a) The difference between the mean temperature of the warmest and the coldest months.
(ii) Albedo	(b) The lines joining the places of equal temperature
(iii) Isotherms	(c) The incoming solar radiation
(iv) Annual range	(d) The percentage of visible light reflected by an object.

Ans. 

(i) Insolation	(a) The difference between the mean temperature of the warmest and the coldest months.
(ii) Albedo	(d) The percentage of visible light reflected by an object.
(iii) Isotherms	(b) The lines joining the places of equal temperature
(iv) Annual range	(a) The difference between the mean temperature of the warmest and the coldest months.

SHORT ANSWER TYPE QUESTIONS

Q.1. Define the term insolation.

Ans. The earth’s surface receives most of its energy in short wavelengths. The energy received by the earth is known as incoming solar radiation which in short is termed as insolation.

Q.2. What are the ways in which the atmosphere is heated?

Ans. The atmosphere gets heated in the following ways:

(a) Radiation.

(b) Conduction.

(c) Convection.

(d) Advection.

Q.3. What are the isotherms?

Ans. These are imaginary lines joining places having equal temperature, reduced to sea level.

Q.4. What part of heat energy radiated by the sun is intercepted by the earth?

Ans. 1/2000 millionth part of the total energy radiated by the sun is intercepted by the earth.

Q.5. What do you understand by the annual range of temperature?

Ans. Annual range of temperature is the difference between the mean temperature of the warmest month and the coldest month.

Q.6. Which is the most important process of heating the atmosphere?

Ans. Terrestrial radiation.

Q.7. Why do isotherms bend while crossing the continents and oceans?

Ans. Due to differential heating and cooling of the land and the oceans.

Q.8. What is air drainage?

Ans. The cold air acts almost like water and moves down the slope to pile up deeply in pockets, and valley bottoms with warm airs above. This is called air drainage. It protects plants from the frost damages.

Q.9. What is temperature?

Ans. The degree of hotness or coldness of a body is called its temperature.

Q.10. Why do different parallels receive different amounts of insolation?

Ans. Different parallels receive different amount of insolation due to the difference of inclination of sun’s rays at different parallels.

Q.11. Why the annual insolation received by the earth on 3rd January is more than that of 4th July?

Ans. The solar output received at the top of atmosphere varies slightly in a year due to the variations in the distance between the earth and the sun. Therefore the annual insolation received by the earth on 3rd January is more than the amount received on 4th July.

Q.12. How do clouds affect the temperature of a place?

Ans. Clouds check the outgoing radiation at night. The temperature does not fall. But on clear nights there is unchecked radiation which escapes rapidly through atmosphere.

SHORT ANSWER TYPE QUESTIONS

Q.1. How does the unequal distribution of heat over the planet earth in space and time cause variations in weather and climate?

Ans. The earth receives almost all of its energy from the sun. The earth in turn radiates back to space the energy received from the sun. In the process the earth neither warms nor cools over a period of time. The amount of heat received by different parts of the earth is not the same. This variation causes pressure difference in the atmosphere. This leads to transfer of heat from one region to the other by winds. Thus the weather and climate are ultimately caused by the unequal distribution of heat over the planet earth over space and time.

Q. 2. Why is the annual range of temperature high in the Siberian plains?

Ans. The mean January temperature between 80°N and 50°N is minus 20°C and the temperature in July is more than 10°C. That is why annual range of temperature is very high.

Q.3. What are the factors that control temperature distribution on the surface of the earth?

Ans. The factors controlling the temperature distribution on the surface of the earth are:

1. The latitude of the place.

2. Altitude of the place.

3. Distance from the sea.

4. Ocean currents.

5. Local aspects.

Q.4. How does angle of the sun’s rays falling on the ground affect the amount of insolation?

Ans. The amount of insolation reaching the earth’s surface and its effectiveness per unit area depends to a great extent upon the angle of incidence or the inclination of the sun’s rays. Change in the angle of incidence of the sun’s rays causes variation in the amount of solar energy reaching the earth’s surface in two ways:

1. When the sun is overhead its rays are nearly vertical over the surface and are more concentrated. Hence, the intensity of insolation is more. If the angle of incidence is low, rays are oblique. Therefore its spread out is more and the intensity of insolation less.

2. The sun’s rays striking the earth at a small angle of inclination traverse more of atmosphere than rays striking at a high angle. Longer the path, greater. the amount of scattering, reflection and absorption by the atmosphere which reduces the intensity of insolation at the surface. At sunrise the angle of incidence of sun’s rays is low. Hence solar energy received is low. At noon, the angle of incidence is highest and so the solar energy received is high. During evening again the angle of incidence becomes lower than at noon and the solar energy received is low. Hence, the solar energy received on the surface of the earth is directly related with the angle of incidence of the rays of the sun.

Q.5. Explain the latitudinal heat balance.

Ans. Although the earth as a whole maintains a balance between the incoming and outgoing radiation, its distribution on the surface of the earth is not uniform. This is primarily due to the spherical shape of the earth and the revolution of the earth around the sun on its inclined axis. The amount of insolation gradually decreases from the equator towards the poles. Similarly the amount of terrestrial radiation also varies. At latitudes below 40 degrees or more the solar radiation received than is lost to space by the earth. Beyond the 40° latitudes more heat is lost than received.

The area between 40° north and south latitudes should have been getting progressively hotter and the area from 40° latitude to the poles getting cooler progressively. But this is not so. The atmosphere and ocean act against thermal energy that transfer heat from the tropics towards the poles. Due to imbalance of heat, winds and ocean currents are produced. This transfer of heat energy from the surplus zones to deficit zones maintains an overall balance over earth’s surface.

Q.6. In India, why is the day temperature maximum in May and why not after the summer solstice?

Ans. After the winter solstice, the Northern Hemisphere is inclined towards the sun. As a result, the sun’s rays are vertical over the Tropic of cancer. It is on 21st June. So the month of May and June experience maximum temperature. But after the summer solstice, the autumnal equinox starts. Thus the temperature starts to come down.

Q.7. What is meant by the vertical distribution of temperature?

Ans. The study of upper layers of atmosphere with the help of balloons, kites, aeroplanes, radio sounds etc. reveals that the temperature of air goes on falling upwards at the rate of 1° per 165 metres of height. This is called Normal Lapse Rate. The temperature decreases with height because of the:

1. Conduction from Earth’s surface: The atmosphere is heated mainly by the terrestrial radiation. The atmospheric layer immediately overlying the earth’s surface receives the maximum heat. It is, therefore the warmest. But as we go higher, the temperature gradually decreases and air becomes cooler because the increasingly higher layers receive a lesser amount of heat.

2. Difference in Absorption: The lower layers have more of water vapour, dust particles, dense air, etc. than the upper layers. Therefore, lower layers absorb a lot of heat from the earth’s surface and directly from the solar radiations passing through them than the upper layers. That is why lower layers are warmer than the upper layers.

Q.8. What are the general characteristics of isotherms?

Ans. The following are the general characteristics of isotherms:

1. Isotherms run along the latitudes, but they are not parallel to the latitudes.

2. They take sudden bends at land-water edges because of land-water contact.

3. They are drawn at equal spaces which indicate the latitudinal thermal gradient.

4. Isotherms change their positions. In summer isotherms passing over the sea bend towards the equator but in winter they turn away from the equator.

Q.9. What are the favourable conditions for the inversion of temperature?

Ans. The following conditions are favourable for the inversion of temperature:

1. Long nights: During the winter, long nights are favourable for making the surface cold due to rapid outgoing radiation.

2. Clear sky: Under the clear sky the loss of heat by radiation is rapid and unchecked.

3. Calm air: During the calm weather, no air movement is in the atmosphere.

4. Dry air: The dry air does not absorb heat radiation and favours inversion of temperature.

5. Snow covered surface: The frozen surface is a poor conductor of heat and checks the upward flow of heat from the ground.

Q.10. What is the climatic significance of inversion of temperature? Give the points.

Ans. It has much climatic significance:

1. It checks the process of convection and advection of air currents.

2. Dense fog occurs in the valley bottoms.

3. The upper layer of the air becomes dry.

4. Dense smog (smoke + fog) results beneath the inversion layer.

5. Stratus clouds are formed below the inversion layer.

Q.11. What is advection?

Ans. The transfer of heat-through the horizontal movement of air is called advection. It is more important than the vertical movement. In mid-latitudes most of the diurnal variation in day and night are called by advection alone.

Q.12. How does the atmosphere affect the incoming (to the earth) short wave solar radiation? How is the atmosphere affected by the outgoing long wave radiation from the earth?

Ans. The atmosphere is largely transparent to short wave solar radiation. The incoming solar radiation passes through the atmosphere before striking the earth’s surface. Within the troposphere water vapour, ozone and other gases absorb much of the near infrared radiation. Very small suspended particles in the troposphere scatter visible spectrum both to the space and towards the earth’s surface. The red colour of the rising and setting sun and the blue colour of the sky are the result of scattering of light within the atmosphere.

The long wave radiation from the earth is absorbed by the atmospheric gases particularly by the carbon dioxide and the other greenhouse gases. Thus the atmosphere is indirectly heated by these long wave radiations.

Q.13. Describe the variability of insolation at the surface of the earth.

Ans. The amount and the intensity of insolation vary during a day, in a season and in a year. The factors that cause these variations in insolation are:

1. Rotation of earth on its axis.

2. The angle of inclination of the sun rays.

3. The length of the day.

4. The transparency of the atmosphere.

The variations in the duration of the day at different latitudes on solstices are given in the table below:

Length of the day in hours and minutes on winter and summer in the Northern Hemisphere.

Latitude	0°	20°	40°	60°	80°
December 22	12h 00m	10h 48m	9h 8m	5h 33m	0
June 21	12h	13h 12 m	14h 52m	18h 27m	6 months

Q.14. Define daily mean temperature and monthly mean temperature.

Ans. Daily mean temperature: The average of the maximum temperature and minimum temperature recorded during a day is known as daily mean temperature.

Daily mean Temperature:

= Max. Temperature + Min. Temp./2

Example = 40° + 20°/2 = 30°C

Monthly mean temperature: The average of the daily mean temperature of all days of a month is known as monthly mean temperature. Monthly mean temperature of June.

= Sum of the daily mean temp. Of 30 days/30

Q.15. What is global warming? What are its causes ? State its effects.

Ans. Global Warming: The burning of fossil fuels the cultivation of soil large scale industrialisation rapid means of transport and deforestation has caused an imbalance in the atmosphere. These activities are increasing the amount of carbon dioxide. Thus green house effect has raised the average temperature of the earth by 0.5°C. By the year 2000 the earth’s average temperature went up by 2°C. This is called global warming. Global warming is causing a rise in sea levels due to melting of glaciers. It is threatening to submerge many coastal areas.

Q.16. Explain the Green house effect of atmosphere.

Ans. Atmosphere is heated by the radiation from the earth’s surface below. This action is composed to that of a glass house permits radiation to get in but does not allow radiation to escape out. Therefore glasshouse is warmer from inside than outside. Atmosphere also acts like a blanket keeping the earth warm. It is known as green house effect of atmosphere. This is due to presence of carbon dioxide in atmosphere. Carbon dioxide has the ability to absorb earth radiation. The carbon dioxide layer acts as a glass roof of the green house. With the increase in carbon dioxide the temperature of the earth is increasing. Due to this the year 1955 was the hottest year in India during this century.

Q.17. Why are isotherms more irregular in the Northern Hemisphere than in Southern Hemisphere?

Ans. Isotherms are more irregular in the Northern Hemisphere than in the Southern Hemisphere because of the marked contrasts in the distribution pattern of land and water in two hemispheres. Isotherms are generally parallel to the latitudes and the effect of altitude is well indicated. But deviation from this general trend is more pronounced in January than in July, and particularly in the Northern Hemisphere. Northern Hemisphere has larger landmass than the southern one. In January isotherms deviate to the north over the ocean and to the south over the continent. The presence of warm oceanic currents, Gulf Stream and North Atlantic Drift, make the Northern Atlantic ocean warmer and the isotherms bend towards the north over the continents. They bend towards south making the isotherms more irregular.

Q.18. Write short notes on:

(i) Heat Budget.

Ans. Heat Budget: The average temperature of the earth remains rather constant. It has been possible because whatever the amount of heat the earth receives from the sun in the form of short-waves, it radiates back to the atmosphere in the form of long waves. There exists a stage of heat balance or equilibrium between the earth and its atmosphere. This balance of heat is termed as the earth’s heat budget.

Let us assume that total heat received at the top of the atmosphere is 100 units. Approximately 65 units are reflected back to the space even before reaching the earth’s surface. Of these, 6 units are reflected back from the top of clouds and 2 units from the snow and ice-covered areas of the earth. The reflected amount of radiation is called albedo of the earth.

The remaining 65 units are absorbed – 14 units within the atmosphere and 51 units by the earth’s surface. The earth radiates back 51 units in the form of long-wave terrestrial radiation. Of these, 17 units are radiated to space directly and the remaining 34 units are absorbed by the atmosphere, 48 units absorbed by the atmosphere (14 units from insolation + 34 units from terrestrial radiation) are also radiated back into space. Thus the total returning from earth is (17 + 48) 65 units, which were received from the sun. Thus, balances the heat on the earth. This is called heat budget.

(ii) Differential heating of land and water.

Ans. Land and water surfaces show contrasting behaviour in relation to the incoming solar radiation. Land warms and cools more quickly than the water. The sun ray’s penetrate to a depth of only 3 feet in land because it is opaque but they penetrate to greater depths in water because it is transparent. The thin layers of the earth become warm more quickly and also become cold more quickly by emitting heat. On the other land the same amount of insolation has to heat larger volume of water in oceans because of the penetration of the solar rays to a greater depths and thus the temperature of the ground surface becomes higher than the water surface though the amount of insolation received is same in both the cases. There is more evaporation from oceans and hence more heat is spent in this process with the result oceans get less insolation than the land surfaces. The specific heat (the amount of heat needed to raise 1 g of a substance by 1°C) of water is much greater than the land surface as the relative density of water is more than the land surface. Hence more heat is required to raise the same volume of water as compared to landmass. The albedo (reflection) of insolation by water surface is more than the land surface. Thus less amount of insolation is received by the oceans in real terms. In these ways, differential heating of land and water surfaces occurs.

Q.19. Distinguish between:

(i) Insolation and Terrestrial Radiation.

Ans. Insolation: The incoming solar radiation is termed as insolation. It is mostly in the form of short waves. The earth’s surface receives only 1/2000 millionth part of the total energy radiated from the surface of the sun. It raises the temperature of the land and water directly and that of the air indirectly.

Terrestrial radiation: Radiation from earth is called terrestrial radiation. The terrestrial radiation is in the form of long waves which are observed by the components of the atmosphere.

(ii) Advection and Convection.

Ans. Advection: Transfer of heat through horizontal movement of the air is called advection. The air temperature of a place will rise or fall depending upon the winds blowing towards the place is from comparatively a warmer or cooler area.

Convection: Vertical movement of the air is called convection. Convective mechanisms are possible only in liquids and gases. When the air of lower layers gets heated, it expands and owing to the decrease in density, moves upwards.

Q.20. What is global warming? What are the causes? State its effects.

Ans. Global Warming: The burning of fossil fuels the cultivation of soil large scale industrialisation rapid means of transport and deforestation has caused an imbalance in atmosphere. These activities are increasing the amount of carbon dioxide. Thus green house effect has raised the average temperature of the earth by 0.5°C. By the year 2000 the earth’s average temperature went up by 2°C. This is called global warming. Global Warming is causing a rise in sea levels due to melting of glaciers. It is threatening to submerge many coastal areas.

LONG ANSWER TYPE QUESTIONS

Q.1. What are the basic mechanisms of heat transfer? Discuss the importance of the mechanisms with reference to the atmosphere.

Ans. There are four basic mechanisms of heat transfer. They are:

1. Conduction.

2. Convection.

3. Radiation.

4. Advection.

1. Conduction: In this, the heat flows from the hotter zone to the colder zone through matter by molecular activity. When two bodies of unequal temperature are in contact with one another, the molecules of warmer body transfer heat to the molecules of colder body. This process of transfer of heat continues until both bodies attain the same temperature or the contact is broken. This mechanism of transfer of heat is important only in the lower layers of the atmosphere because the ability of the substances to conduct heat varies.

2. Convection: In this mechanism the heat is transferred from hotter zone to the colder zone by the movement of a mass or substance. Convective motions are possible only in liquids and gases.

When the air of lower layers of the atmosphere gets heated by terrestrial radiation or conduction, it expands. Owing to decrease in density, it moves upwards. It pushes the air to higher level which moves upward. The vacuum created by the wind causes horizontal movement in the air. Thus, the wind moving horizontally on the earth’s surface reaches the heated region from where it again ascends. In this way, the heat is transferred from the lower layers to the upper layers of the atmosphere. This mechanism of heating is known as Convection.

3. Radiation: Radiation means the transfer of heat through a medium without affecting it. So the atmosphere does not get heat from the rays of the sun but it is heated by the terrestrial radiation.

4. Advection: In this mechanisms, there is horizontal movement of the air masses. Thus these air masses transfer heat from equatorial regions to the polar regions.

Out of these mechanisms of heat transfer, conduction is the least important meteorologically because air is fluid and is a poor conductor of heat. It has its value in the lower layers of the atmosphere where the air is in direct contact with the earth’s surface.

Q.2. Why do different parallels receive different amounts of insolation?

Ans. The different parallels receive different amounts of insolation. The distribution pattern of insolation is given below:

1. The maximum amount of insolation is received at the equator and it goes on decreasing towards poles.

2. Along the parallel of 45° latitude, it is only about 75% of that at the equator.

3. Along the parallels of 66½°, it is about 50% of that at the equator.

4. At poles, it is about 40% of that at the equator.

In this way, we see that maximum amount of insolation is received at the equator and minimum at the poles.

Why it is so? Because the distribution of insolation over the earth’s surface is controlled by two major factors:

1. The angle of incidence of sun rays: In course of a year, the angle of incidence is more higher at the equator than the poles. It gives more heat to the equator. The angle of incidence is smaller at the poles. Hence it gives less heat to the poles.

2. The duration of sunshine or length of the day: The duration of sunshine in equatorial region is more than the polar regions. Equator and poles receive 350 and 150 thermal days respectively.

Thus the above two factors do not remain same at each parallel line, they vary from parallel.

Q.3. Discuss the factors controlling the horizontal distribution of temperature especially with reference to July and January condition.

Ans. The horizontal distribution of temperature is not uniform on the earth’s surface because of the following factors:

1. Latitude: As a general rule, temperature decreases from the equator to the poles. Because, the sun rays fall more obliquely as we go north or south of the equator. The oblique rays spread on a bigger area than the perpendicular rays and also pass thicker layers of the atmosphere where their heat is absorbed by water-vapour, dust particles and carbon dioxide.

2. Land-water contrasts: It is another important factor affecting the horizontal distribution of temperature. The land gets heated and cooled down more rapidly than the seas. Thus, the contrasts between land-sea temperature affects the horizontal distribution of temperature and isotherms take sudden bends at lands-water edges.

3. Ocean currents: The ocean currents also have great influence on the horizontal distribution of temperature. The warm currents increase the temperature while cold currents reduce it in the coastal regions.

4. Winds: Cold and warm winds also affect the horizontal distribution of temperature. Warm winds increase the temperature whereas the cold winds reduce it.

January Isotherms: Isotherms are more irregular in Northern Hemisphere and more regular in Southern Hemisphere. The isotherms are closely spaced in the Northern Hemisphere. ‘More than 30°C’ isotherm passes through north- west Argentina and central Australia in the Southern Hemisphere.

July Isotherms: ‘More than 30° temperature occurs in the belts between 10° and 40° altitude. The isotherms bend towards equator on the ocean towards pole on continents and vice versa in the Southern Hemisphere. Isotherms are more regular in Northern Hemisphere.

Q.4. Distinguish between normal lapse rate and the inversion of temperature.

Ans. Normal Lapse Rate: The atmosphere is heated mainly by terrestrial radiation. Hence, the lower layer of the air in contact with the earth’s surface is the warmest and temperature gradually decreases with the increasing height. On the average there is a fall of 1°C per 165 metres of ascent. This is known as normal lapse rate of temperature.

Inversion of Temperature: Inversion of temperature means the upsetting of the general rule of vertical distribution of temperature, i.e. temperature decreases with the increasing height. In this phenomenon, the temperature increases with the increasing altitude. Hence, the colder layers of the air are located near the surface of the earth and warmer layers of the air are high above.

Inversion of temperature occurs in a calm, cold and clear winter night because, the earth’s surface cools more rapidly and the layer immediately overlying the earth’s surface also gets cooled to a greater degree than the higher layer. Hence as the temperature instead of decreasing, increases with the increasing height. This type of inversion of temperature is termed as surface inversion of temperature.

On clear and calm nights it is generally experienced in valleys when the radiation has caused cooling and the cold air has shunk down. At the same time, on the mountain slopes the air is warmer.

Favourable conditions for inversion of temperature: Following conditions favour the development of surface inversion of temperature:

1. Long nights.

2. Clear skies.

3. Snow covered surface.

4. Slight air movement.

5. Flattish surfaces with a few topographic depressions.

Q.5. How do the latitude and the tilt in the axis of rotation of the earth affect the amount of radiation received at the earth’s surface?

Ans. Effect of latitude: The amount of insolation received by a place on the surface of earth depends upon the angle of inclination of the rays. The angle of inclination of the sun rays depend upon the latitude of a place. The higher the latitude, the less is the angle the rays make with the surface of the earth resulting in slant rays. The area covered by the vertical rays is always less than the slant rays. If more area is covered, the energy gets distributed and the net energy received per unit area decreases. Moreover, the slant rays are required to pass through greater depth of the atmosphere resulting in more absorption, scattering and diffusion. Hence the high latitudinal areas get less net insolation as compared to the low latitudinal areas.

Effect of the tilt in the axis of rotation of the earth: If all the other conditions are favourable to a place on the earth’s surface then longer duration of sunshine or day of the length and shorter duration of night enable the place to receive more amount of insolation. The duration of sunshine or the day of length varies at all places except at the equator due to the tilt

in the inclination in the axis of rotation of the earth (66½°). The length of day is almost 12 hours on the equator because the light circle always divides the equator in two equal halves. The length of day increases poleward with the northward march of the sun in Northern Hemisphere while it decreases in the Southern Hemisphere at the time of summer solstice (21 June). In contrast the length of day increases from the equator poleward in the Southern Hemisphere but it decreases in the Northern Hemisphere at the time of winter solstice. The following table describes this variation in the length of day on solstices due to the tilt in the axis of rotation of the earth.

Latitude	0°	20°	40°	60°	90°
December 22	12h 00m	10h 48m	9h 8m	5h 33m	0
June 21	12h	13h 12m	14h 52m	18h 27m	6 months

Q.6. Discuss the processes through which the earth-atmosphere system maintains heat balance.

Ans. The following process are involved in heating the atmosphere:

(i) Conduction: Conduction is the process in which the heat flows from a region of higher temperature to region of lower temperature through contact. The land absorbs insolation and warm up the above air slowly. As air bring a gas has a very low conductivity conduction is not a major process of heating the atmosphere.

(ii) Convection: Convection is the most important process of heating the atmosphere. It is a process of transferring the heat from a hotter to colder region through vertical descending or ascending air currents. When the air of lower layer gets heated it expends the rises upward as ascending winds. The colder air descends down to take its place on the earth. A cycle circulation of convection currents helps in distributing the heat of lower layers to upper layers.

(iii) Terrestrial Radiation: The atmosphere is mostly heated by out going long wave earth radiation. The earth absorbs 51% of short wave solar energy. The earth surface radiates this energy as long wave solar energy. The heating up of atmosphere through this outgoing radiation is called the process of radiation.

(iv) Advection: Advection means horizontal transfer of heat by winds from a hotter to colder region. The air temperature of a place will rise if the air mass is coming from a warmer area. A cold air mass will reduce the temperature of a warmer region.

(v) Compression: When the mass of the air descends it is being compressed in volume as a result of compression the temperature in lower layer is increased.

(vi) Heat budget: The average annual temperature of the earth is a whole is neither decreasing nor increasing. It remains almost constant. The incoming solar radiation is balanced by an equal amount of out going radiation returned from the earth. A balance is maintained between the insolation and the radiation returned from the earth. This is known as heat budget.

Q.7. Compare the global distribution of temperature in January over the Northern and Southern Hemisphere of the earth.

Ans. The temperature distribution is generally shown on the map with the help of isotherms, the lines joining places having equal temperature. The isotherms are generally parallel to the latitude. The deviation from this general trend is more pronounced in the month of January, especially in the Northern Hemisphere. As the land surface area is much larger in the Northern Hemisphere compared to that in the Southern Hemisphere, the effects of landmass and ocean currents are more pronounced.

In January, the isotherms deviate to the north over the ocean and to the south over the continents. This can be seen on the North Atlantic Ocean. The presence of warm ocean current, Gulf stream and North Atlantic drift make the North Atlantic ocean warmer and the isotherms bend towards the north. Over the land the temperature decreases sharply and the isotherms bend towards south in Europe. The effect of the ocean is quite evident in the Southern Hemisphere. Here the isotherms are more or less parallel to the latitudes and the variation in temperature is more gradual than in the Northern Hemisphere. The isotherm of 20°C, 10°C and 0°C run parallel to 35°S, 45°S and 60°S latitudes, respectively.

Q.8. What is inversion of temperature? When and in what regions does it take place?

Ans. At times, the situations are reversed and the normal lapse rate is inverted. It is called Inversion of temperature. Inversion is usually of short duration but quite common nonetheless. A long winter night with clear skies and still air is ideal situation for inversion. The heat of the day is radiated off during the night, and by early morning hours, the earth is’ cooler than the air above. Over polar areas, temperature inversion is normal throughout the year. Surface inversion promotes stability in the lower layers of the atmosphere.

Smoke and dust particles get collected beneath the inversion layer and spread horizontally to fill the lower strata of the atmosphere. Dense fogs in mornings are common occurrences especially during winter season. This inversion commonly lasts for few7 hours until the sun comes up and beings to warm the earth. The inversion takes place in hills and mountains due to air drainage.